Schematic view of the 1D geometry used in this work. The code solves the chemical evolution of a slab of gas flowing at a constantvelocity V_gas and irradiated from the left (upstream) along a direction parallel to the flow. The proton density profile n_H (z) is prescribedand the temperature T_K is constant. The radiation field is the sum of a FUV part modeled by an ISRF scaled by G₀ and a visible part modeled by a black-body radiation field at T_vis = 4000 K, which is diluted by a factor W (see Appendix A). The attenuation of the radiation field by gas-phase photoprocesses and by dust, if any, along z was consistently computed.